Knife gate valves have traditionally been used to control the flow of heavy slurries transported through tubular conduits. The gate has a blade portion with a beveled edge to cut through the material suspended in the slurry as the gate is lowered to seal off the flow through the valve. The gate typically closes against the metal valve body, and the slurry assists in sealing the valve.
A problem associated with using a knife gate to control the flow of slurries of light viscosities is that the material accumulating on the surface of the valve seat does not promote a tight seal. Knife gate valves controlling the flow of light viscosity slurries typically employ, therefore, a sealing member that is made of a resilient substance. This member is squeezed between the gate and the metal valve body when the valve is in the closed position, thereby forming the seat of the valve and sealing the valve. The material accumulating on the sealing member tends, however, to cause the gate to adhere to and pull out the sealing member when the gate is either retracted to open the valve or extended to close the valve. This is especially true for valves that control the flow of nonlubricating fluids. Lubricating the gate contacting surface of the valve sealing member can reduce surface friction and thereby prevent adhesion of the gate to the seat. Lubricating the surface of the sealing member of a knife gate valve precludes, however, its use in conduits that transport substances such as, for example, air and natural gas, which would be contaminated by a lubricating substance. In the transportation of more viscous slurries, moreover, the flow of the slurry when the gate is open may be sufficient to dislodge the sealing member and carry it downstream.